1. Field of the Invention
The present invention relates to a method of producing a member having a periodic structure finer than a wavelength and an optical element formed of the member. According to the present invention, by forming an artificially controlled fine structure, a base bulk material for forming an optical element having an optional (or desired) refractive index distribution or optical anisotropy can be produced.
2. Related Background Art
Hitherto, an optical crystal or an organic film has been used as means for obtaining an optional refractive index distribution or optical anisotropy. For example, a low-pass filter having a plurality of quartz crystal plates or LiNbO3 thin plates adhered to each other, a wavelength plate or a deflecting plate having an optically anisotropic film sandwiched by optical glass sheets, and the like have been put to practical use. These are bulk materials and are therefore suitable for various kinds of processing and can be handled relatively easily.
However, in the conventional low-pass filter using a crystal, in the case where quartz crystal is used, the difference between refractive indexes of an ordinary ray and an extraordinary ray is small, so that it is necessary to increase the thickness of the crystal in order to obtain a sufficient performance as an optical element. It is, therefore, difficult to reduce the size of the low-pass filter. On the other hand, LiNbO3 has a large difference between refractive indexes of an ordinary ray and an extraordinary ray, so that the thickness thereof has to be reduced more than necessary, which makes its processing difficult and requires particular attention in the handling thereof.
Further, because the conventional film-type phase plate or wavelength plate is made of an organic material, there has been a problem that the high-temperature resistance is poor. Therefore, for example, when used as a component in a projector such as a liquid crystal projector, it is unsuitable for use as an optical component provided in the vicinity of an optical source the temperature of which becomes high.
On the other hand, in recent years, in accordance with development of a fine processing technique, it has become possible to produce a fine structure of a submicron order. At present, the methods of producing an element of a fine structure can be classified into the following three types.
A first-type method is a method of stacking thin layers with different refractive indexes using a film formation technique such as vapor deposition or sputtering. According to this method, there is a possibility that a bulk material may be made by stacking an extremely large number of layers.
A second-type method is a method of using exposure, development and etching as is the case with the method of producing a diffraction grating or the like, which is the so-called photolithography. By this method, it has recently become possible to produce a structure finer than a wavelength as the techniques of exposure and etching have advanced. In addition, with the photolithography, since it is possible to distribute and form desired patterns having different shapes on a plane, an artificial refractive index distribution can be formed two-dimensionally. On the other hand, since there is a limitation in the thickness, it is difficult to make a bulk material.
A third-type method is a method of producing a fine structure having a directionality by oblique vapor deposition technique. With this method, a thin film having structural birefringence can be made without using an advanced fine processing technique.
Those elements with fine structures that are produced by the above-described methods are constituted of a metal or a dielectric material and have a high-temperature resistance. In addition, concerning the optical function, since they are designed independently, even when it is difficult to handle an obtained element, it is possible to determine specifications freely by design change or the like. For example, a member with a fine structure produced by the first-type method can be applied to an interference film which is used so as to make a light incident perpendicularly on its film surface and have hitherto been applied to various fields. In the case where the member is used as such an interference film, it is sufficient that a maximum number of layers to be stacked is approximately one hundred, although depending upon the desired optical characteristic.
However, in order to develop refractive index anisotropy in a member having such a fine structure, it is necessary to use the member with a light being made incident on a film horizontally. Thus, for example, in the case where the member is used as a CCD optical component of approximately ⅓ inch, a size of approximately 6 mm square is required. When such a member is to be produced by the first-type method, because the formed thin film structure has a thickness of one layer of approximately 100 nm at the most, the process results in a multilayer structure having as many as 60,000 layers. In this case, even if one layer can be formed in ten seconds, it takes about one week to produce the member. In addition, in case of a thin film, an internal stress tends to be generated when formed. Due to this internal stress, it is difficult to produce a thin film structure with a thickness of several mm.
On the other hand, the second-type method is for obtaining an optional pattern on a substrate through steps of film formation, resist application, exposure, development, etching, and peeling off (see, for example, Japanese Patent Application Laid-Open No. 7-191209). With this method, although the structure of a film material can be processed optionally by etching, the etching has a limitation in a depth. In particular, in the case where it is desired to obtain a line/space finer than a wavelength, a complete rectangular structure cannot be obtained due to an influence of side etching, edge deposition, or the like. In addition, even if the complete rectangular structure is obtained, it has a depth in the order of several xcexcm at the most and cannot be a bulk material.
Further, concerning a production apparatus to be used in the second-type method, an expensive stepper is required for performing exposure of a fine structure. Further, since an expensive apparatus such as an RIE or an ICPE is required for etching as well, the production cost will increase.
On the other hand, with the third-type method, since relatively inexpensive vapor deposition process is used, and since structural birefringence can be obtained simply by carrying out film formation obliquely, it can be said that this method is very effective. In this method, a high refractive index material is a vapor deposition material and a low refractive index material is the air as voids.
However, in the third-type method, although the size of the voids is controlled by controlling the incident angle of vapor deposition particles on a substrate and vapor deposition conditions, the size can only be increased by approximately 30% at the maximum. In addition, since the variation of size is also large, the effect of structural birefringence decreases and, at the same time, lowering in the transmission efficiency due to light scattering also occurs. Further, it is difficult to make a film thick as in the first-type method.
It is, therefore, an object of the present invention to solve the above-described prior art problems and provide a method of producing a member which has a periodic structure finer than a wavelength, inexpensively and easily.
Further, it is another object of the present invention to provide an optical element such as a phase plate or a low-pass filter which can be produced inexpensively and easily by processing the member produced by the above-described method.
According to the present invention, there is provided a method of producing a member of a periodic structure having a plurality of high refractive index layers and a plurality of low refractive index layers having a refractive index lower than a refractive index of the high refractive index layers, alternately stacked. Each of the high refractive index layers and low refractive index layers has a thickness finer than a wavelength. The method comprising the steps of:
alternately superimposing a plurality of first sheet glasses having a high refractive index and a plurality of second sheet glasses having a refractive index lower than a refractive index of the first sheet glasses on each other to form a stacked member;
heating the stacked member to a temperature of not less than a glass transition temperature; and
applying a pressure to the heated stacked member perpendicularly to a principal surface of the sheet glasses or extending the heated stacked member parallel to the principal surface of the sheet glasses, thereby integrating the stacked member while reducing the thickness of each of the sheet glasses.